Method for estimating the torque of a heat engine

ABSTRACT

A method estimates a torque of a heat engine in a vehicle hybrid transmission including at least a heat engine and an electric machine together or separately supplying a heat engine torque and heat engine torque intended for wheels of the vehicle. The method uses a measurement of a speed of the heat engine, a value of the heat engine torque reference, and a value of the electric machine torque. The method also sums an estimate of a total torque supplied by the transmission to the wheels and of an estimate of an equivalent resistive torque of the transmission to determine the estimated heat engine torque.

The present invention relates to the field of heat engine control.

More specifically, its subject is a method for estimating the torque of a heat engine in a vehicle hybrid transmission comprising at least a heat engine and an electric machine together or separately supplying a heat engine torque and an electric torque intended for the wheels of the vehicle.

This method can be applied to any heat engine or hybrid powertrain having two rotating shafts (or pinions) that need to be synchronized in order to engage a transmission ratio.

Torque control of a heat engine in a hybrid or non-hybrid vehicle is of capital importance in improving the overall performance of the vehicle and the drivability thereof.

When the gearbox associated with the engine is a parallel shafts gearbox comprising at least a primary shaft connected to a power source and a secondary shaft driven by the primary shaft in order to transmit the motive torque to the wheels, it requires good control over the (heat engine and/or electric) torque in order to avoid potential jerkiness in the torque curve as torque is reapplied after the changes in gear ratio. Control over the motive torque during the changes in gear ratio is of particular importance in certain hybrid architectures in which the synchronizing of the two gearbox shafts, prior to engaging a gear ratio, is assigned to the heat engine.

However, the measurement of the torque of the heat engine is not directly available in a vehicle moving along. One means for obtaining its value is to estimate (reconstruct) it indirectly from measurements of the angular rotational speed of the crankshaft.

Publication FR 2 681 425 discloses a method for measuring the torque of an internal combustion heat engine using the signal produced by a sensor associated with the engine flywheel ring gear. This method makes it possible to calculate the mean torque produced by each combustion of the gaseous mixture in each cylinder of the engine. The calculated values can be used to continuously improve engine operation and monitor defects thereof. The engine management computer is capable of adapting to the empirical measurements taken from the flywheel ring gear. Continuous improvement of combustion performance is performed by loop control of the combustion parameters, this all assuming good knowledge of the dynamics of the combustion systems and the response times thereof.

This method is somewhat unsatisfactory in complex environments such as hybrid architectures, because of the combined effects of the inertias and frictions within the powertrain. Each motive power source, heat engine and electric machine, actually has its own dynamics and its own level of response specific to the control instructions.

When the two shafts (pinions) that are to be coupled are synchronized by way of the heat engine, the latter needs moreover to meet the driver's demand for torque. The control system therefore demands very accurate information regarding the instantaneous value of the heat engine torque. It is notably necessary for the discrepancy in speed between the shafts that are to be synchronized to converge very quickly to a range of 30 revolutions per minute in order for the gearshift to be acceptable, with a speed differential that is as small as possible. The phase that follows on from the coupling of the two shafts (reapplication of torque) also needs to be transparent, which means to say to take place with the least possible amount of jerkiness.

It is an object of the present invention to reconstruct a torque signal produced by the heat engine, taking account of its transmission to the gearshift control members and to the wheels.

In particular, the invention seeks to allow robust control over engine speed during the synchronization phase prior to the engaging of a gear ratio, when this synchronization is performed by the heat engine. The estimated torque signal needs to be sufficiently accurate that gear shifts can be performed in a manner that is transparent to the user.

To this end, the invention proposes to estimate the heat engine torque as being the sum of an estimate of the total torque supplied by the transmission to the wheels, and of an estimate of the overall resistive torque of the transmission.

For preference, this method uses a measurement of the speed of the heat engine, the value of the heat engine torque reference, and the value of the electric machine torque.

The invention will be better understood from reading the following description of one nonlimiting embodiment thereof and by referring to the attached drawing, the single FIGURE of which illustrates the key steps in the method.

By applying the fundamental principle of mechanics to a hybrid powertrain comprising a heat engine and an electric machine, the following dynamic model is obtained:

-   -   for the heat engine: J_(th){dot over (ω)}_(th)=T_(th)−T_(th)         ^(res)     -   for the electric motor: J_(em){dot over (ω)}_(em)=T_(em)−T_(em)         ^(res)     -   for the wheel: J_(r){dot over (ω)}_(r)=T_(r)−T_(r) ^(res)         where:     -   ω_(th): the speed of the heat engine,     -   T_(th): the torque of the heat engine,     -   T_(th) ^(res): the resistive torque of the heat engine,     -   J_(th): the inertia of the heat engine,     -   ω_(em): the speed of the electric motor,     -   T_(em): the motor torque,     -   T_(em) ^(res): the resistive torque of the electric motor,     -   the inertia of the electric motor,     -   ω_(r): the speed of the wheel,     -   T_(r): the torque applied to the wheel,     -   T_(r) ^(res): the resistive torque applied to the wheel (unknown         input of external origin),     -   J_(r): the inertia of the wheel.

The torque of the heat engine T_(th) is always a response to the driver's torque demand (reference) T_(th) ^(ref). If τ is the symbol used for a time constant of the heat engine (τ (comprised between τ_(min) and τ_(max)) being indicative of the responsiveness of the heat engine to achieving its torque reference T_(th) ^(ref), this can be written:

τ{dot over (T)} _(th) =T _(th) ^(ref) −T _(th).

Whatever the kinematic mode of the transmission, the overall inertia of the powertrain can be referenced to the heat engine by introducing the notions of “equivalent inertia” or “inertia with respect to the heat engine” J_(eq-em), and of equivalent resistive torque T_(eq) ^(res).

The fundamental principle of dynamics, applied to the sum of the driving and resistive torques of the transmission, can be written as follows: J_(eq) _(_) _(th){dot over (ω)}_(th)=T_(th)+αT_(em)−βT_(eq) ^(res), where (α) and (β) are dependent on the stepdown gear ratios between the heat engine shaft and the wheels. β is dependent in particular on the stepdown gear ratios of the gearbox and on the axle assembly of the vehicle.

From a relationship of this type it is possible to determine the value of equivalent inertia of the transmission with respect to the heat engine J_(eq-th). The equivalent resistive torque may be the resistive torque applied to the wheel T_(r) ^(res) or the resistive torque of the engine or motor. From this equation, the invention proposes constructing an “observer” that makes it possible to establish an estimate of the heat engine speed {circumflex over (ω)}_(th), an estimate {circumflex over (T)}_(th) of the torque T_(h), applied by the heat engine, and an estimate {circumflex over (T)}_(eq) ^(res) of the equivalent resistive torque {circumflex over (T)}_(eq) ^(res), guaranteeing the “robustness” of {circumflex over (T)}_(th).

The method is illustrated in FIG. 1. It makes it possible to estimate the torque of a heat engine in a vehicle hybrid transmission comprising at least a heat engine and an electric machine together or separately supplying a heat engine torque T_(th) and an electric torque T_(m) intended for the wheels of the vehicle. The output signals {circumflex over (ω)}_(th), {circumflex over (T)}_(th), {circumflex over (T)}_(eq) ^(res) may be seen in FIG. 1, together with the estimate of the total torque {circumflex over (T)}_(th) ^(tot). The estimated torque of the heat engine {circumflex over (T)}_(th) is the algebraic sum of {circumflex over (T)}_(th) ^(tot) and of {circumflex over (T)}_(eq) ^(res). These estimates are produced from just three inputs used by the observer: a measurement of the speed of the heat engine ω_(th), the value of the heat engine torque reference T_(th) ^(ref), and the value of the electric machine torque T_(em).

M₀, M₁, k₁ and k₂ are the gains that need to be calibrated. A first gain M₀ or “drift compensation” is assigned the sign of the difference (ω_(th)-{circumflex over (ω)}_(th)) to be added to the product α·T_(em). This sum is integrated in order to give the estimate {circumflex over (ω)}_(th) of the engine speed with loop correction by the product of the integration with the inverse of the equivalent inertia J_(eq-th). The term M₀ assigned the sign of (ω_(th)-{circumflex over (ω)}_(th)) is multiplied by the inverse of the gain k₁. This product is integrated, then corrected with the result of the integration x₁. To sum up, an estimate {circumflex over (ω)}_(th) of the engine speed is obtained from its measurement ω_(m) by integrating the sum of the product α·T_(em) and of the calibrated gain M₀ assigned the sign of the difference ω_(th)-{circumflex over (ω)}_(th).

The sign of x₁ is imposed on the second gain M₁, used to calculate the torque estimates {circumflex over (T)}_(th) ^(tot), {circumflex over (T)}_(th) and {circumflex over (T)}_(eq) ^(res) in the subsequent steps. The term M₁·sign(x₁) is added to the torque reference T_(th) ^(ref) to give, through integration, the estimate of the total torque {circumflex over (T)}_(th) ^(tot). This undergoes double integration after having been multiplied in succession by the inverse of the gain k₂ and by the inverse of the engine time constant τ. The estimate of the equivalent resistive torque {circumflex over (T)}_(eq) ^(res) is obtained by multiplying the result by the inverse of the parameter β mentioned above. As indicated in the figure, the estimate of the heat engine torque {circumflex over (T)}_(th) is the sum of the estimate of the total torque {circumflex over (T)}_(th) ^(tot) and of the equivalent resistive torque {circumflex over (T)}_(eq) ^(res).

The proposed method for estimating the torque thus breaks down into two main phases:

-   -   a first phase that involves estimating the speed of the heat         engine {circumflex over (ω)}_(th), the total applied torque         {circumflex over (T)}_(th) ^(tot) and the equivalent resistive         torque {circumflex over (T)}_(eq) ^(res), and     -   a second phase that involves estimating the applied torque of         the heat engine {circumflex over (T)}_(th) from the total         applied torque {circumflex over (T)}_(th) ^(tot) and from the         equivalent resistive torque {circumflex over (T)}_(eq) ^(res).

In order to estimate the speed of the heat engine {circumflex over (ω)}_(th), the total applied torque {circumflex over (T)}_(th) ^(tot) and the equivalent resistive torque {circumflex over (T)}_(eq) ^(res), the observer has available to it only the heat engine speed measurement ω_(th), the torque reference T_(th) ^(ref) and the electric machine torque T_(em).

In the second step, the equivalent resistive torque {circumflex over (T)}_(eq) ^(res) is multiplied by the parameter β.

In conclusion, the invention proposes a robust method for estimating torque for a vehicle equipped with a heat engine or hybrid powertrain. The observer allows the applied torque of the heat engine {circumflex over (T)}_(th), the total applied torque {circumflex over (T)}_(th) ^(tot) and the equivalent resistive torque {circumflex over (T)}_(eq) ^(res) to be estimated. This estimation makes it possible for the next step, that of coupling the two shafts (reapplication of torque or torque switchover) to be rendered transparent, thereby considerably reducing the jerkiness of the torque curve. The method notably allows better control over how well the torque curve is followed while two shafts are being synchronized, by means of the heat engine. Finally, the convergence of the estimated speed {circumflex over (ω)}_(th) onto the measured speed ω_(th) is somewhat insensitive to variations in the parameters of the system, such as the inertia and response time of the actuator, or any lags there might be, so that this observer is particularly robust. 

1-6. (canceled)
 7. A method for estimating a torque of a heat engine in a vehicle hybrid transmission comprising at least a heat engine and an electric machine together or separately supplying a heat engine torque and heat engine torque intended for wheels of the vehicle, comprising: using a measurement of a speed of the heat engine, a value of the heat engine torque reference, and a value of the electric machine torque, and summing an estimate of a total torque supplied by the transmission to the wheels and of an estimate of an equivalent resistive torque of the transmission to determine the estimated heat engine torque.
 8. The method for estimating the torque as claimed in claim 7, wherein a value for transmission equivalent inertia with respect to the engine is determined from a relationship of the type J_(eq) _(_) _(th){dot over (ω)}_(th)=T_(th)+αT_(em)−βT_(eq) ^(res), where parameters α and β are dependent on the stepdown gear ratios between a heat engine shaft and the wheels, T_(th) is the heat engine torque, T_(em) is the electric machine torque, ω_(th) is the speed of the heat engine, {circumflex over (T)}_(eq) ^(res) is the equivalent resistive torque, and J_(eq-th) is the transmission equivalent inertia with respect to the engine.
 9. The method for estimating the torque as claimed in claim 8, wherein an estimate of engine speed is obtained from a measurement of the engine speed by integrating a sum of a product of the parameter a and the electric machine torque and of a calibrated gain, to which is assigned the sign of the difference of the measurement of the speed of the heat engine and the estimate of the engine speed.
 10. The method for estimating the torque as claimed in claim 9, wherein the integrated term is corrected in a loop by its product with an inverse of the equivalent inertia.
 11. The method for estimating as claimed in the torque as claimed claim 7, further comprising estimating, in a first phase, the speed of the heat engine, the total applied torque, and the equivalent resistive torque, and estimating, in a second phase, the applied torque of the heat engine from the total applied torque and from the equivalent resistive torque.
 12. The method for estimating torque as claimed in claim 7, wherein the equivalent resistive torque is multiplied by the parameter β which is dependent on stepdown gear ratios of a gearbox and of an axle assembly of the vehicle. 